I. Field
The present invention relates generally to data communication, and more specifically to techniques for performing frequency error estimation and frame synchronization in an orthogonal frequency division multiplexing (OFDM) communication system.
II. Background
OFDM is a multi-carrier modulation technique capable of providing high performance for some wireless environments. OFDM effectively partitions the overall system bandwidth into multiple (Nsb) orthogonal subbands, which are also commonly referred to as tones, sub-carriers, bins, and frequency channels. With OFDM, each subband is associated with a respective sub-carrier that may be modulated with data.
In an OFDM system, a transmitter initially codes, interleaves, and modulates a stream of information bits to obtain a stream of modulation symbols. In each OFDM symbol period, Nsb “transmit” symbols can be sent on the Nsb subbands, where each transmit symbol can be a data symbol (i.e., a modulation symbol for data), a pilot symbol (i.e., a modulation symbol for pilot), or a signal value of zero. The transmitter transforms the Nsb transmit symbols to the time domain using an inverse fast Fourier transform (IFFT) and obtains a “transformed” symbol that contains Nsb time-domain chips. To combat frequency selective fading (i.e., a frequency response that varies across the Nsb subbands), which is caused by multipath in a wireless channel, a portion of each transformed symbol is typically repeated. The repeated portion is often referred to as a cyclic prefix and includes Ncp chips. An OFDM symbol is formed by the transformed symbol and its cyclic prefix. Each OFDM symbol contains NL chips (where NL=Nsb+Ncp) and has a duration of NL chip periods, which is one OFDM symbol period (or simply, “symbol period”). The transmitter may transmit the OFDM symbols in frames, with each frame containing multiple (Nsym) OFDM symbols. The frames of OFDM symbols are further processed and transmitted to a receiver.
The receiver performs the complementary processing and obtains NL samples for each received OFDM symbol. The receiver removes the cyclic prefix from each received OFDM symbol to obtain a received transformed symbol. The receiver then transforms each received transformed symbol to the frequency domain using a fast Fourier transform (FFT) and obtains Nsb “received” symbols for the Nsb subbands, which are estimates of the Nsb transmit symbols.
The receiver typically performs frequency error estimation to determine the frequency error at the receiver. The frequency error may be due to a difference in the frequencies of the oscillators at the transmitter and receiver, Doppler shift, and so on. The receiver also typically performs frame synchronization to detect for the start of each frame so that a proper sequence of received symbols can be provided for demodulation, deinterleaving, and decoding.
To support frame synchronization, the transmitter typically transmits a training sequence across each frame. This training sequence contains pilot symbols and is transmitted on designated subbands. The receiver processes the training sequence to detect for the start of each frame. The training sequence represents overhead that reduces the efficiency of the system. Moreover, detection performance based on the training sequence is typically not robust, especially at low signal-to-noise ratio (SNR) conditions.
There is therefore a need in the art for techniques for performing frequency error estimation and frame synchronization in an OFDM system.